TPi July 2015

A number of different light sources were trialled, with a series of thin, high-intensity white LED array bars mounted on the testing machine’s crosshead and bed being the best. These provide enough light to enable the camera system to be run with very low gain settings, with the additional benefit of flooding the sample with enough illumination to reduce the effects of external light fluctuations. Further trials on the camera-based system showed that painting the anti-buckling device in ‘blackboard’ paint, with extremely low levels of reflectivity, helped to reduce the likelihood of the camera system accidentally targeting a reflection instead of the markers, and produced a cleaner signal. In both cases, verification work has shown that OSM systems offer an accurate, cost-effective alternative to electrical resistance bonded strain gauges, especially as there is no mechanical degradation when the test material is subjected to cyclic plastic strain. For the laser system, signal quality when compared with strain gauges can be more open to influence from external sources such as airflow and ambient light fluctuations. However, the implemented set-up has ensured that signal quality is well within acceptable limits to provide the levels of accuracy required. The RTSS proved to be less sensitive to the surrounding environment. It is also quicker and less complex to set up, and can be transported between test rigs with ease. The comparative flexibility of these modules has meant that it has been possible to utilise them in multiple testing activities.

Timing and costs are very important factors in testing. Both systems offer quicker test coupon preparation when compared to bonded strain gauges, requiring only the removal of surface mill scale, spraying with low reflection paint and installing contrasting adhesive backed markers where gauges would usually be located. Laser system The first system tested was laser based. The apparatus used a pair of laser light source generators, one aimed at the outer ‘cap’ surface of the test coupon and the other aimed at the inner ‘root’ surface, to scan the sample. As the light source passes from a black painted surface over a white marker, the reflected light is diffused. Changes in diffusion are tracked and software interprets each scan of the light source, which is then compared with the previous image and changes in elongation or compression between them calculated. The resulting data is converted into a real-time strain output for real-time analysis. Trials undertaken compared the results of dummy test coupons, instrumented with both strain gauges against laser system markers and also calibrated extensometers. Post- verification work proved that the laser system measures strain to within, at most, 0.05% to a magnitude of 3% strain. While these figures were considered to be accurate, some challenges were identified during the trials. Some variation in precision, due to changes in ambient light conditions and local temperature fluctuations, was noted, and precautions had to be taken for future testing to avoid these influences. Following the investigation, trial and verification process, finalised system specifications were provided to the manufacturer by the Exova Daventry team. After a final shakedown, the complete system was installed in the Exova Spijkenisse laboratory, and has been in use in the Netherlands since 2012. Real Time Strain Sensor (RTSS) system The second system trialled was a camera-based module. Named Real Time Strain Sensor (RTSS), it consists of two high-resolution cameras aimed at the test coupon and operating at a frame rate of approximately 200Hz. The system identifies significant contrast change in order to determine the position of the markers. This is similar in principle to the marker detection process of the laser system. The software tracks the pixel change between markers during loading by comparing the latest frame to the previous frame. This then converts the change into a real-time strain output. A major difference between the systems is the need for much greater light in the area on which the RTSS camera is focussed. The use of an ‘anti-buckling device’, which encases the test coupon during compressive loading, shields the markers from ambient light, so an external light source is required.

Laser system

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July 2015 Tube Products International